The present invention relates generally to an exposure apparatus used to manufacture semiconductor integrated circuit (“IC”) devices, etc., and more particularly to maskless exposure that dispenses with a photo-mask or reticle as an original. This invention is suitable for defect monitoring in an exposure apparatus that uses an optical modulator (also referred to as a spatial light modulator).
A projection exposure apparatus has been conventionally used to expose a mask pattern onto a substrate on which a photosensitive agent is applied in manufacturing a semiconductor device and a liquid crystal panel. However, as the finer processing to the mask pattern and a larger mask size are demanded with the improved integration and increased area of the device, an increase of the mask cost becomes problematic. Accordingly, the maskless exposure that dispenses with the mask for exposure has called attentions.
One exemplary attractive maskless exposure is a method for projecting a pattern image onto a substrate using a phase-modulation type optical modulator. The optical modulator is a parallel-connected type device, and the number of pixels per unit time may possibly be increased enormously. The phase modulation needs a minute displacement of a mirror, and thus is suitable for high-speed operation. In particular, a grating light valve (“GLV”) type optical modulator that uses a modulated pattern of a diffraction grating is suitable for a large amount of data transfers, and a maskless exposure apparatus that transfers enormous data amount. The maskless exposure apparatus that uses the optical modulator instead of the mask to modulate the exposure light in accordance with a desired pattern, and condenses the pattern via a projection optical system, and forms the pattern on the substrate. GLV is disclosed, for example, in Optics Letters, Vol. 17, pp. 688-690 (1992).
Referring now to FIGS. 10A and 10B, a description will be given of an operational principle of a conventional GLV 20. Here, FIG. 10A shows a relationship between the section of the GLV 20 and a phase difference when the GLV 20 turns off. FIG. 10B shows a relationship between the section of the GLV 20 and a phase difference when the GLV 20 turns on.
Each element in the GLV 20 has a pair of catoptric bands or ribbons 21, and each pixel 23 includes three elements 22. The GLV 20 is a reflection-type phase modulator that has plural pixels 23 arranged in parallel. One of ribbons 21 in each element 22 is connected to a switch (not shown), and configured to vary its level, for example, when the voltage is applied to it.
In operation, when the switch turns off as shown in FIG. 10A, all the ribbons 21 have the same level. When the switch turns on, as shown in FIG. l0B, the ribbons 21 fall alternately by a quarter of the irradiation wavelength, and the reflected light have a phase difference of 180° between two adjacent ribbons 21. When the switch turns off, only the 0th order diffracted light is reflected since the reflected light is reflected while its phase is not modulated. On the other hand, when the switch turns on, the reflected light is phase-modulated and the ±1st order diffracted lights are reflected
Referring now to FIG. 11A, a description will be given of control over the diffracted light using the GLV 20. Here, FIG. 11A is a schematic view for explaining the control over the diffraction light using the GLV 20. As shown in FIG. 11A, a filter 32 that blocks the 0th order light is provided between a lens 31 and the GLV 20. When the switch turns off, no light is incident upon the lens 31. When the switch turns on, the ±1st order diffracted lights are incident upon the lens 31. A maskless exposure apparatus that controls the exposure light is configured when it installs the GLB 20 instead of the mask and the lens 31 is regarded as the projection optical system.
Other prior art include J. W. Goodman, Introduction to Fourier Optics 2nd ed., ISBN 0-07-114257-6.
In the maskless exposure, the optical modulator generates a pattern instead of a mask pattern, and it is important to confirm whether the optical modulator properly generates the pattern. Of course, that the final device operates properly is a proof of the normal exposure. However, if the device does not work, it is difficult to identify whether the cause rests on the exposure or another process or thus to improve the yield. Therefore, it is preferable to confirm whether the optical modulator properly generates a pattern during the exposure.
There are roughly two causes that deteriorates the pattern generation, i.e., 1) an offset from a set pattern due to a data transfer error or a GLV's malfunction (this cause is referred to as a “defect” hereinafter); and 2) the light intensity shortage due to the contamination on the GLV (this cause is referred to as an “uneven screen light intensity” hereinafter). The defect results in the unexpected pattern generation and the light intensity shortage, and thus the final device is highly likely inoperable. The uneven screen light intensity appears as a critical dimension (“CD”) scattering in the screen, and deteriorates the device performance. In particular, the reduced CD scattering is demanded strictly in the cutting-edge semiconductor device manufacturing technology, and the uneven screen light intensity should also be corrected for the future application of the maskless exposure apparatus to the state-of-the-art field.
When these causes occur during exposure, a check of the optical modulator and corrective or auxiliary exposure is needed if the defect or uneven screen light intensity is correctable. With a non-correctable defect, the disposal of the device is needed or the resist should be applied again to restart the exposure. It is necessary for the corrective exposure to know the information as to the level of the drop of the light intensity in the area that requires a correction, and the light intensity distribution in the overall exposure area. This information should be acquired at the exposure time for each exposure position.